A PLANAR DODECASUBSTITUTED PORPHYRIN

Structural investigations of copper and nickel complexes of dodecasubs tituted porphyrins bearing aryl groups at the meso positions and propa no rings at the pyrrole beta positions reveal considerable differences in their macrocycle conformations. While the nickel complex NiTC5T(3, 4,5-OMeP)P was found to exhibit a nonplanar conformation which is cons iderably more planar than that of other dodecasubstituted porphyrins, the corresponding copper complex CuTC5T(3,4,5-OMeP)P was planar. CuTC5 T(3,4,5-OMeP)P thus represents the first example of a completely plana r dodecasubstituted porphyrin. The crystal structures of both porphyri ns reveal that the Cb-Cb-CH2 angle is 13-degrees smaller than in OEP d erivatives. This change, which moves the methylene and aryl substituen ts further apart, effectively removes the steric repulsion responsible for the very nonplanar conformations observed for other dodecasubstit uted porphyrins. Molecular mechanics calculations using a porphyrin fo rce field correctly predict a planar macrocycle conformation. The poss ible reasons for the discrepancy between the observed moderately nonpl anar structure and the calculated planar structure for NiTC5T(3,4,5-OM eP)P are discussed. The usefulness of spectroscopic probes (NMR, reson ance Raman, electronic absorption) in predicting the planarity of dode casubstituted porphyrins is also examined. The identification of a pla nar dodecasubstituted porphyrin further indicates the flexibility of t he tetrapyrrole macrocycle and has implications for the study of nonpl anarity in synthetic porphyrins and metallotetrapyrrole containing bio molecules. Crystal data: NiTC5T(3,4,5-OMeP)P.2CH2Cl2 (C68H68N4NiO12.2C H2Cl2), triclinic, P1BAR, a = 14.492(8) angstrom, b = 14.453(8) angstr om, c = 17.689(10) angstrom, alpha = 103.11(5)-degrees, beta = 113.58( 4)-degrees, gamma = 89.93(5)-degrees, V = 3290(3) angstrom3, Z = 2, la mbda(mo Kalpha) = 0.7107 angstrom, T = 130 K, R = 0.06 for 7657 reflec tions with I > 2.5sigma(I); CuTC5T(3,4,5-OMeP)P.CH2Cl2 (C68H68N4CuO12. CH2Cl2), monoclinic, P2(1)/c, a = 14.459(4) angstrom, b = 15.302(6) an gstrom, c = 14.879(4) angstrom, beta = 100.68(4)-degrees, V = 3235(2) angstrom3, Z = 2, lambda(Mo Kalpha) = 0.7107 angstrom, T = 130 K, R = 0. 106 for 4222 reflections with I > 2.0 sigma(I).